Fuel cells are classified into a phosphoric acid one, a molten carbonate one, a solid oxide one, a polymer electrolyte one and the like based on types of electrolytes composing the fuel cells. Among them, with regard to the polymer electrolyte fuel cell (hereinafter, referred to as a “PEFC”), a device thereof is small and an output thereof is high in comparison with the fuel cells of the other modes. Accordingly, the PEFC is placed as a system that plays a dominant role in the next generation. The PEFC is expected to serve as a fuel cell for small-scale on-site power generation, for a power source of a movable body such as a vehicle, for a portable instrument, or for the like.
In a basic structure of the PEFC, gas diffusion electrodes on which a catalyst such as platinum is supported are arranged on both sides of a proton (hydrogen ion) conducting membrane, and a pair of separators having a structure to supply a fuel are arranged further on both outsides of the arranged gas diffusion electrodes. This basic structure is taken as a unit cell, and a plurality of the cells adjacent to one another are coupled to one another, whereby it becomes possible to take out a desired power. For example, when hydrogen is supplied as the fuel from one side (generally referred to as an anode or a fuel electrode) of such an assembly, a reaction of H2→2H++2e− occurs on the fuel electrode side by the catalyst, and protons and electrons are generated. Here, the protons are supplied to an opposite electrode (generally referred to as a cathode, an air electrode or an oxygen electrode) side through the electrolyte membrane (referred also to a proton conducting membrane) in contact with electrodes of the fuel electrode and air electrode sides. Moreover, the electrons are collected at the electrode on the fuel electrode side, are used as electricity, and are thereafter supplied to the air electrode side. Meanwhile, the air electrode (oxygen electrode) side receives air (oxygen) supplied thereto, the protons which have passed through the proton conducting membrane, and the electrons used as the electricity, and a reaction of ½O2+2H++2e−→H2O occurs in the presence of the catalyst.
As described above, chemical reactions under the fuel cell operation occur on interface portions between the proton conducting membrane and the gas diffusion electrodes on which the catalyst is supported, and accordingly, interface structures of the membrane, the electrode and the catalyst largely affect performance of the fuel cell, which is such as power generation efficiency. The assembly of the proton conducting membrane and the gas diffusion electrodes is generally referred to as a membrane-electrode assembly (MEA), and has become one of major technology development fields of the fuel cell.
In the MEA, it is necessary that the membrane, the catalyst and the electrodes are coupled to one another while having appropriate interfaces thereamong. Specifically, when the fuel electrode side is taken as an example, it is necessary that the hydrogen or the like as the fuel can contact a catalyst surface, and that the protons and the electrons, which are generated from the hydrogen, are efficiently delivered individually to the membrane and the electrodes. At present, the one used most normally as the proton conducting membrane for the fuel cell is sulfonated fluorine resin (representative example: trade name “Nafion (registered trademark)” made by DuPont Corporation) having thermoplasticity.
However, the sulfonated fluorine resin having the thermoplasticity has a problem of being short of heat resistance at the time of operating the polymer electrolyte fuel cell. Specifically, the sulfonated fluorine resin exerts proton conductivity in such a manner that ion channels are formed therein by coagulation of sulfone groups; however, the sulfonated fluorine resin has a defect of being plastically deformed at a specific temperature or more because of having the thermoplasticity, resulting in the breakage of an ion channel structure. Therefore, in the sulfonated fluorine resin, the plastic deformation occurs in a short time at approximately 130° C. of a glass transition temperature (Tg) or more, and gradually occurs even in a temperature range from 100 to 130° C., and ion conductivity is lowered, whereby it is difficult to maintain high fuel barrier property.
Moreover, in recent years, there have also been examined fuel cells, each using fuel such as alcohol, ether and hydrocarbon, which is other than the hydrogen, as the fuel of the fuel cell, and taking out the protons and the electrons from the fuel other than the hydrogen by the catalyst. A representative example of the fuel cells as described above is a direct methanol fuel cell (DMFC) using methanol (in usual, used as an aqueous solution) as the fuel. The DMFC does not require an external reformer, and easily handles the fuel, and accordingly, is expected most as a small and portable power supply among various types of the fuel cells.
However, the above-mentioned sulfonated fluorine resin membrane has extremely high affinity with the methanol, and accordingly, has had a serious problem that the sulfonated fluorine resin concerned largely swells by absorbing the methanol, causes a so-called methanol crossover in which the methanol permeates the swelled proton conducting membrane to then leak out to the cathode side, resulting in that the output of the fuel cell is largely lowered.
Meanwhile, also with regard to electrolyte membranes for the fuel cell, which are made of other than the sulfonated fluorine resin, various types of membranes and the like, which are of hydrocarbon series, inorganic series and the like, are developed actively. For example, an organic silicon compound is composed of a silicon-oxygen bond having strong bonding energy, and accordingly, has high chemical stability, high heat resistance and high oxidation resistance, and can impart many exceptional properties depending on a composition thereof. Therefore, the organic silicon compound is used in every industrial field such as electrics, electronics, business machines, construction, food, health care, textile, plastic, paper, pulp, paint and rubber.
A proton conducting membrane is disclosed, which uses this organic silicon compound and has a cross-linked structure composed of the silicon-oxygen bond (for example, refer to Patent Literature 1). Even in the case of being exposed to a high-temperature and high-humidity environment under strong acidic conditions (where the protons are present) as in the proton conducting membrane, the cross-linked structure composed of the silicon-oxygen bond is relatively stable, and can be suitably used as a cross-linked structure in an inside of the fuel cell membrane. Moreover, even in the case of using the alcohol such as the methanol, the swelling is suppressed to be small by the silicon-oxygen cross-linked structure, and the methanol crossover can be expected to be reduced.
On the other hand, a solution of conventional sulfonated fluorine resin with thermoplastisity dissolved in a polar solvent such as alcohol as an electrode binder, and a paste (catalyst ink) of the electrode binder mixed with an electron conductive material such as carbon supported with catalyst such as platinum is coated on a gas diffusion layer such as a carbon paper, and dried; the resultant electrode has generally been used. Here, the electrode binder is a liquid to adjust a catalyst ink so as to be liquid-like or paste-like by mixing with an electron conductive material supported with catalyst. In order to form an electrode, a catalyst ink is dried and solidified, and a layer, called a catalyst layer, containing a catalyst supported electron conductive material and an electrode binder solidified material (binding component), is formed.
Here, as the electrode binder, the following three points are mainly required; 1) High affinity for electrode constituent components. 2) High proton conductivity after the electrode catalyst layer is formed by drying and solidifying. 3) After solidifying, a strength not destroyed by a variation of temperature environment or a pressure due to a flow of gas or liquid, is added to the catalyst layer, and its strength is maintained for a long time.
In case of using the electrode composed of the electrode binder solidified material of the sulfonated fluorine resin described above, a softening due to heat and a swelling due to methanol absorption of the sulfonated fluorine resin are large, and accordingly in the operation under high temperatures, and/or in case of using a fuel of a relatively high methanol concentration, there have been problems, for example, a sufficient power generation performance can not be obtained or durability is deteriorated.
Consequently, when a catalyst layer of the electrode is fabricated by using a cross-linked compound having a silicon-oxygen bond and an electrode binder containing a polymer material having an acid group, a softening due to heat and a swelling due to methanol are inhibited and a high power generation performance can be expected to be realized.
However, when even higher power generation performance is required to be obtained, as an acid component, whether a material has a specific property inherent in an inorganic material or inherent in an organic material, when the material has a large molecular weight as well as an almost single molecular weight distribution, then, since an acid component is difficult to enter inside of a secondary agglomerate (agglomerate) of catalyst supported carbon composing the electrode, a utilization efficiency of catalyst such as platinum and the like is insufficient, whereby problems such as an output decrease and the like have been produced.
Furthermore, in case of fabricating MEA using the proton conductive membrane having the silicon-oxygen cross-linked structure described above and the electrode of sulfonated fluorine resin used as a electrode binder solidified material (binding component), since their constituent components are largely different with each other, there have been the case where adhesion strength between the membrane and the electrode becomes weak (for example, refer to Patent Literature 2).
Even in this case, when an electrode binder solidified material composing the electrode is formed by using the electrode binder containing the cross-linked compound having the silicon-oxygen bond and the polymer material having an acid group, which are described above, MEA with high membrane-electrode adhesion strength as well as with high durability can be expected to be obtained.
Moreover, in the proton conducting membrane having the silicon-oxygen cross-linked structure, intermolecular force of a silicon-carbon bond in the membrane to the peripheries thereof is somewhat weaker than that of the silicon-oxygen bond, and impact resistance of this proton conducting membrane to an external pressure derived from a humidity change, sudden swelling and the like is sometimes low. Therefore, for example, in the case of using a certain type of the organic silicon compound as a material of the membrane-electrode assembly for which high proton conductivity under high and low temperature conditions is required, performance deterioration in the proton conductivity and the fuel barrier property, which is caused by temperature variations, sometimes occurs. In this connection, an MEA using a membrane having high impact resistance while having the cross-linked structure has been required.
(Cited Literatures)
Patent Literature 1: Japanese Patent No. 3679104
Patent Literature 2: International Publication WO03/026051